The use of compact disk (CD) data storage is well known in the art. CDs comprise a data layer residing beneath a clear coating layer having a well defined thickness. The data layer has marks of varying reflectance which are read by a laser beam focused on the data layer. The laser beam must be focused to a spot of minimal size in order for the system to achieve maximum data density, which is desired.
In order for the reading laser beam to be focused to a spot of minimal size, the optical system which focuses the laser beam must be designed to compensate for the distorting effects of spherical aberration. Spherical aberration has the effect of enlarging the focused spot size, which generally precludes achieving maximum data density.
The amount of spherical aberration, however, varies with the thickness of the clear coating layer. For CDs having a single data layer and a clear coating of a known thickness, an objective lens assembly can be designed to compensate for the well defined magnitude of spherical aberration present. An optical focusing system designed for a given clear coating thickness cannot be used on a CD having a substantially different clear coating thickness.
It is expected that future optical data storage systems may use multiple data layers located at different depths. Such a system will require an adjustable spherical aberration compensation capability. The spherical aberration compensation will need to be different for each data layer.
More generally, spherical aberration requires correction in any system which must focus light to a minimal spot size at various depths within a material. Such systems are not limited to the current optical data storage systems.
U.S. Pat. No. 5,202,875 to Rosen et al. discloses an optical data storage system using multiple data layers. Different embodiments of the invention exploit different methods of providing spherical aberration correction. Stepped plates having well defined thicknesses disposed between the objective lens and CD are used in one embodiment. The stepped plates have different thicknesses and are moved in and out of the optical path such that the light always passes through the same thickness of material (clear coating material) before hitting the data layer. Paired wedges and rotatable stepped wedges also perform the same function.
Another aberration compensation technique disclosed by Rosen in the above referenced patent 5,202,875 involves the use of two lenses (a convex lens and a concave lens) in addition to the objective lens assembly. The lenses are moved relative to one another to provide controlled spherical aberration correction of arbitrary magnitude. The distance between the lenses determines the magnitude of aberration correction. Rosen also discloses using an aspheric lens with zero focal power. The compensator of Rosen is characterized in that the spherical aberration compensation lens disclosed by Rosen must be located in a beam being focused, i.e. a noncollimated beam.
U.S. Pat. No. 5,610,901 to Best et al. discloses substantially the same spherical aberration compensation techniques as the 5,202,875 patent to Rosen et al.
U.S. Pat. No. 5,157,555 to Reno discloses a spherical aberration correction apparatus which uses two lenses having an adjustable air gap between them. The lenses are convex and concave, with complementary surfaces facing each other. Reno's apparatus is used in conjunction with an objective lens assembly for focusing the beam onto the data surface. Reno uses lenses which have a nonzero focal power (large spherical curvature) and so the spherical aberration compensation lenses tend to alter the distance between the objective lens and focal point as spherical aberration is adjusted. The focus adjustment and spherical aberration adjustment are coupled together, thereby complicating the adjustment procedure.